BMI calculator
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- Reviewed
Body mass index is body weight in kilograms divided by height in metres squared. An adult weighing 70 kg at 1.75 m has a BMI of 22.9, which the World Health Organization classifies as a healthy weight. The WHO adult bands are underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above.
Result
Body mass index — healthy weight
22.9
BMI = weight(kg) ÷ height(m)²
- WHO category
- Healthy weight
- Healthy weight range
- 56.7 kg – 76.3 kg
- Healthy BMI band
- 18.5 – 24.9
A person weighing 70.0 kg at a height of 175 cm has a BMI of 22.9, which the World Health Organization classifies as healthy weight for adults. The healthy-weight range for that height is 56.7 kg to 76.3 kg.
BMI is a population screening measure, not a diagnosis. It does not distinguish muscle from fat and is not valid for children, pregnancy, or highly muscular adults.
How to use the BMI calculator
- 01
Choose your units
Select metric for kilograms and centimetres, or imperial for pounds, feet and inches. The calculator converts internally and reports the same BMI either way.
- 02
Enter your weight
Type your body weight. Measure first thing in the morning, before eating, for the most consistent figure.
- 03
Enter your height
Type your standing height without shoes. Height loss with age is real, so use a current measurement rather than one from years ago.
- 04
Read your category
The result strip shows the BMI value and the WHO category, and the rows beneath give the healthy weight range for your height.
The formula
BMI = weight(kg) ÷ height(m)² BMI = 703 × weight(lb) ÷ height(in)²
- 703
- The conversion constant that makes the imperial formula produce the same kg/m² value.
- BMI
- Body mass index, expressed in kg/m².
- weight(kg)
- Body mass in kilograms.
- height(m)
- Standing height in metres.
The index was published by Adolphe Quetelet in 1832 as a population statistic and was named body mass index by Ancel Keys in 1972. It was designed to describe groups, not to assess individuals — a limitation that still applies.
Worked example
- Units
- Metric
- Weight
- 70 kg
- Height
- 175 cm
- Result
- BMI 22.9 — healthy weight
Convert height to metres: 175 cm ÷ 100 = 1.75 m. Square it: 1.75 × 1.75 = 3.0625 m². Divide weight by that figure: 70 ÷ 3.0625 = 22.9 kg/m². That value falls inside the WHO healthy band of 18.5 to 24.9. For this height, the healthy band corresponds to a weight range of 56.7 kg to 76.3 kg, or 125 lb to 168 lb.
Frequently asked questions
What is a healthy BMI range for adults?
The World Health Organization defines the healthy adult range as a BMI of 18.5 to 24.9 kg/m². Below 18.5 is classified as underweight, 25.0 to 29.9 as overweight, and 30.0 and above as obese, subdivided into class I at 30 to 34.9, class II at 35 to 39.9, and class III at 40 and above.
Is BMI accurate for muscular people?
No. BMI measures total mass against height and cannot distinguish muscle from fat. Muscle is denser than fat, so trained athletes frequently register as overweight or obese despite low body fat. For anyone carrying substantial muscle, a waist-to-height ratio or a body fat estimate gives a far more informative picture.
Does BMI work for children and teenagers?
Not with adult cut-offs. Children's body composition changes continuously with age, so paediatric assessment uses BMI-for-age percentiles from CDC or WHO growth charts rather than fixed thresholds. A child at or above the 95th percentile for their age and sex is classified as obese; between the 85th and 95th, overweight.
Do BMI thresholds differ by ethnicity?
Yes. Cardiometabolic risk rises at lower BMI values in people of South Asian, Chinese and some other Asian ancestries. The WHO expert consultation identified 23 kg/m² as a public-health action point and 27.5 kg/m² as a high-risk threshold for these populations, and several national guidelines including NICE in the United Kingdom apply lowered cut-offs.
What is the healthy weight range for a given height?
Multiply the height in metres squared by 18.5 and by 24.9 to obtain the lower and upper bounds. At 1.75 m, height squared is 3.0625, so the healthy range runs from 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg — approximately 125 lb to 168 lb.
What is a better measure of health risk than BMI?
Waist circumference and waist-to-height ratio track abdominal fat, which drives cardiometabolic risk more directly than total mass. Keeping waist circumference below half of height is a widely used rule. Body fat percentage from a DEXA scan or bioelectrical impedance adds composition detail that BMI structurally cannot capture.
Is BMI valid during pregnancy?
No. BMI is not a valid measure during pregnancy, because gestational weight gain reflects the fetus, placenta, amniotic fluid and expanded blood volume rather than body composition. Clinical guidance instead uses pre-pregnancy BMI to set recommended gestational weight gain ranges, which are monitored by an obstetric provider.
Sources
- Obesity and overweight — BMI classification — World Health Organization
- About Adult BMI — US Centers for Disease Control and Prevention
- Appropriate body-mass index for Asian populations — WHO Expert Consultation, The Lancet
Last reviewed: · Formula and sources verified by Syed Aqeel Ahmad Gillani. See the methodology for how every calculation is derived.